1. Field of the Invention
The present invention is generally in the field of amplifiers. More specifically, the invention is in the field of power amplifiers for communication devices.
2. Related Art
A continuing desire to significantly improve the talk time of cellular handsets using various wireless communication standards, such as code-division multiple access (“CDMA”), time-division multiple access (“TDMA”), and global system for mobile communications (“GSM”), has led to the development of high efficiency power amplifiers. To achieve a further reduction in battery current consumption, many power amplifiers used in cellular handsets have been designed to operate efficiently, not only at maximum output power in a high-power mode, but also at lower output power in a low-power mode. In the low-power mode, low power consumption is often achieved by using a mode-control circuit to reduce current consumption by reducing bias voltage applied to the power amplifier. Although the mode-control circuit reduces the current consumption of the power amplifier, the mode-control circuit causes only a minimal reduction in the power gain of the power amplifier. As a result, the mode-control circuit fails to provide a sufficient reduction in the power output level of the power amplifier in the low-power mode to meet a minimum power specification.
In one attempt to achieve a power amplifier having the necessary low current consumption and power gain reduction required to meet the minimum power specification, device manufacturers have controlled the reference voltage (“Vref”) applied to the power amplifier to reduce gain while utilizing mode-control circuitry to reduce the power amplifier's current consumption. Although controlling Vref to reduce power gain and utilizing mode-control circuitry to reduce current consumption allows the power amplifier to meet the minimum power specification in low-power mode, the circuitry required to control Vref is difficult to implement.
In another attempt to meet the minimum power specification in low-power mode, one of more stages of the power amplifier can be bypassed to sufficiently reduce the power gain of the power amplifier. However, this approach requires an additional switch, which undesirably increases circuit complexity. A further attempt utilizes an external attenuator to reduce the power gain of the power amplifier sufficiently to meet the minimum power specification in the low-power mode. However, this approach requires additional components, which undesirably increase manufacturing cost and complicate the design process of a cellular handset utilizing the power amplifier.
Thus, there is a need in the art for a means for reducing power gain in a power amplifier, such as a power amplifier used in a cellular handset, that is effective, easy to implement, and cost effective.